Composite propellant containing nitroglycerin



3,265,543 CDMPOSXTE PROPELLANT CGNTAKNING NETRGGLYdCEREN Aibert SmithCarter, New Castle, DeL, assignor to I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware No Drawing. Fiied June 28, 1962, Ser. No.205,876

6 Qlairns. (53!. 149-19) This invention relates to composite propellantcompositions. More particularly this invention relates to newplasticized polymeric binder compositions, to composite propellants madewith such binders, and to processes for preparing these compositions.

Solid propellant compositions for propelling rockets and the like arebecoming increasingly important because of certain inherent advantagespossessed by these propellants as contrasted with liquid propellants.Prominent among solid propellants are compositions designatedcollectively as composite propellants which are mixtures of fuel andoxidizer in separate phases, the oxidizer or the oxidizer and part ofthe fuel being finely divided solid inorganic material, and theremaining part or all of the fuel being an organic material of plastic,resinous or elastic nature. The organic fuel acts also as a binder tohold the finely divided solid components in a Well mixed and uniformstate of distribution and to confer tensile and compressive strengthsand a sufficient degree of elasticity that the finished solid propellantcompositions will withstand handling, transportation, storage, andfiring Without significant deformation or fragmentation. The term binderas used herein includes the plastic, resinous, and elastic matrixmaterials of composite propellants which hold the composition togetherand which have fuel value in the propellant composition.

In the manufacture of such solid propellants, the resinlike fuel-bindersor their components are mixed in the fluid state with the oxidizer andother ingredients, thus making possible the shaping of the propellantcomposition by extrusion, rolling, casting or other processes, eventhough the propellant composition is composed predominantly ofparticulate solid material. After being thoroughly mixed and shaped, asby being extruded or being placed in a mold, the mixture is cured duringa period of storage, often at an elevated temperature. The shapedplastic mass sets up to a solid during the process.

The fluid state of the binder may be achieved by incorporating it intothe mixture at an elevated temperature, subsequent solidification beingachieved upon cooling the composition. Or the binder may be formed byincorporating liquid intermediates which subsequently react to form thepolymer in situ during the aging or storage period. Alternatively, a lowmolecular Weight fluid prepolymer is incorporated during mixing, andthis is further polymerized to form the solid binder during the aging orcuring period after mixing.

Solvents for a resin-like binder may be employed to increase fluidityand facilitate mixing, the solvent being removed during the curingperiod, though generally with less satisfactory results because of thedifiiculty of removing the solvent from massive shapes of the propellantcomposition.

Another technique used for incorporating a binder into a compositepropellant composition depends upon the use of a plastisol, that is, aliquid dispersion of finely divided resin in a plasticizer. When themixed composition subsequently is stored or heated, the plasticizersolvates the resin particles and the mass gels into a more or less rigidstructure, the physical properties of which are dependent upon the kindand amount of plastisol ingredients incorporated in the propellantcomposition.

Several materials have been proposed and used as States ate binders inthe manufacture of composite propellants. Generally these are organicpolymers chosen from among several types such as those named anddescribed in Chapter 25, p. 287, of Rocket Propellant Handbook, by B.Kit and D. S. Evered, published by the Macmillan Co., New York, 1960.All such :polymeric organic binders are characterized by a relativelyhigh fuel value which requires a large proportion of inorganic oxidizingagent for development of the maximum amount of energy. Thus, theinorganic oxidizer may comprise to and even more of the weight of thecomposite propellant, and other additives about 5%, leaving only 10 to20% of the composition as binder. The relatively small proportion ofbinder which is permitted in a composite composition designed formaximum propellant effectiveness may cause formulation problems andoften leads to marginal physical properties in the finished composition.

Attempts have been made to increase the effectiveness of a polymericorganic binder as a matrix forming material by including in the totalcomposition components which act as plasticizer for the polymericmaterial and thereby improve the ease both of mixing and of shaping thetotal propellant composition. However, since such plasticizers alsogenerally are organic materials of high fuel value, their inclusion doesnot permit a significant reduction in the proportion of granular solidoxidizer to be incorporated in the composition, and hence does notconvey a maximum improvement in the processing and performancecharacteristics of the composition.

Polyurethanes are recognized as desirable binders because of their goodphysical properties and the valuable properties which they confer oncomposite propellants formulated with them. The polyurethanes, however,suffer from the drawback mentioned above, viz., the high requirement ofsolid oxidizer to be used with them. For these, as for most otherpolymeric binders, there is need for a means of reducing the proportionof solid oxidizer which often contributes friability to the finishedpolyurethane-containing composition and excludes the possibility ofincorporating high energy additive fuels such as finely divided metalsof low atomic number.

Incorporation of an energy-rich, fluid, oxygen-bearing plasticizer inthe polyurethane binder composition offers a means of overcoming thedifficulties described above. Since such a plasticizer requires noinorganic oxidizer and, in fact, itself contributes oxygen to themixture, at corresponding reduction in the proportion of solid inorganicox-idizer in the composition is practicable. The resulting increasedproportion of binder matrix increases the flexibility and toughness ofpropellant grains without destroying the essential fuel-oxidizer balancethat produces the maximum effective impulse during the combustion whichoccurs upon firing the propellant composition in a rocket.

An additional benefit can be realized by use of an effective oxidizingplasticizer in the binder composition. If, in contrast to the conditiondescribed above, additional binder matrix is not required in thecomposite propellant, the displaced solid inorganic oxidizer may bereplaced by a suitably balanced blend of an inorganic oxidizer and afuel additive such as a finely divided metal from the group comprisingboron, aluminum, and magnesium, of which aluminum especially ispreferred. The high heat of combustion of the metal, say aluminum,further increases both the specific impulse and the density impulse ofthe propellant composition beyond that attainable in the absence of themetal fuel additive.

Nitroglycerin for many years has been used as a component ofconventional double base propellants, i.e., those containing principallynitrocellulose and nitroglycerin. Nitroglycerin is an effectiveenergy-rich plasticizer for 1D nitrocellulose and is especially valuablebecause of its high density and also because it is an oxygen-richplasticizer. Attempts have been made to incorporate nitroglycerin intoother polymeric organic binders, but generally with little or onlylimited success because the nitroglycerin is poorly compatible andsooner or later gradually exudes from the composition, thereby resultingin changes in the composition and in the properties and performance ofthe propellant, and additionally creates a relatively hazardouscondition due to localized accumulations of nitroglycerin.

In accordance with this invention, the difliculties encounteredheretofore can be overcome by preparing as a binder for a compositepropellant a plasticized mixture of nitroglycerin and a thermoplasticelastic copolymer consisting essentially of the recurring units (a) OGO,(b) O-XO, and (c) Y connected by the bivalent acyl radical wherein O-G-Ois a bivalent radical obtained by removing the hydroxyl hydrogen atomsfrom a polymeric glycol having a molecular weight of at least 800 andselected from the group consisting of polyalkyleneether glycols andpolyester glycols; O-X-O is a bivalent radical obtained by removing thehydroxyl hydrogen atoms from a glycol having a molecular weight of lessthan 250; and Y is a bivalent hydrocarbon radical having a molecularweight of less than 200 and selected so that the polypiperazine amide,having repeating units of the structure in the fiber-forming molecularweight range melts above 200 C.; said recurring units (a), (b), and (c)being present in amounts such that the molar ratios of a:b:c are1.0:0.0-3.0:0.02.0 and the sum of b+2c is equal to or greater than 2.Thermoplastic elastic copolymers of the kind defined above contain amultiplicity of urethane structural units and hence fall within thegeneral category of modified polyurethanes, and are furthercharacterized by a high degree of compatibility with nitroglycerin. Thatis, the polymer is solvated by the nitroglycerin plasticizer to form asolidified or semi-solidified composition from which the nitroglycerinis not lost by exudation even after long storage at elevatedtemperatures.

In compositions of the instant invention, the G in the recurring (a)units is derived from glycols which may be represented by the generalformula HOGOH and have a molecular weight of at least 800. Usefulpolyalkyleneether glycols are represented by the formula HO (RO) Hwherein R is an alkylene radical containing up to carbon atoms and n isan integer such that the molecular weight of the polyalkyleneetherglycol is at least 800. Representative examples of these glycols arepoly-1,2- propyleneether glycol, ethylene oxide modifiedpolypropyleneether glycol, polytetramethylene ether glycol andpolytetramethylene formal glycol. Of these glycolspolytetramethyleneether glycol of 800-3500 number average molecularweight is preferred. The polyalkyleneether glycols are made by thepolymerization of cyclic ethers such as ethylene oxide, propylene oxidesand tetrahydrofuran or by condensation of low molecular weight glycols.

The polyester glycols which are useful in the present invention have anacid number of less than 2 and a number average molecular weight of atleast 800. They may be prepared by condensation polymerization of adicarboxylic acid with more than one mole of an organic diol. The excessof diol determines the molecular weight of the polyester glycolproduced. Representative examples of diols which may be used to preparepolyester glycols for use in the present invention include ethyleneglycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol,thiodiglycol, diethylene glycol and triethylene glycol. Aliphaticdicarboxylic acids such as succinic acid, glutaric acid, adipic acid,sebacic acid, and maleic acid, are generally preferred, but smallamounts of aromatic dicarboxylic acids such as phthalic acid andterephthalic acid may also be used. Anhydrides of dicarboxylic acids maybe conveniently employed in place of the acids when they are available.

The X in O-XO (unit (b) above) is a bivalent radical obtained byremoving the hydroxyl groups from a glycol having a molecular weight ofless than 250 and a general formula of HOXOH. Representative glycolsinclude ethylene glycol, 1,2-propylene glycol, 1,3- propylene glycol,1,4-butanediol, thiodiglycol, diethylene glycol,2,2-dimethylpropanediol-1,3, 2-methyl-2-ethylpropanediol-1,3,cyclohexanediol-1,4, 1,4-dihydroxymethylbenzene,1,4-dihydroxymethylcyclohexane, 1,5-pentanediol and 1,10-decanediol.

The Y in unit (c) above is a bivalent hydrocarbon radical having amolecular weight of less than 250 which may be considered as beingderived from a dicarboxylic acid having the structure 0 O Hoi JY0-" 0HRepresentative dicarboxylic acids include succinic acid, glutaric acid,adipic acid, suberic acid, terephthalic acid, maleic acid andcyclohexane-1,4-dicarboxylic acid.

By nitroglycerin is meant the explosive liquid nitrate ester, glyceroltrinitrate, and also the various mixtures of glycerol trinitrate andother nitrate esters such as ethylene glycol dinitrate commonly employedin the explosive industry.

Oxidizers suitable for use in composite propellants of the instantinvention include lithium perchlorate, sodium perchlorate, potassiumperchlorate, ammonium perchlorate, ammonium nitrate, sodium nitrate, andpotassium nitrate, the perchloratesespecially ammonium perchlorate-beingpreferred.

Fuels in addition to the plasticized binder, sometimes designated asfuel additives, may be included in said composite propellants. Suitablefuels include finely divided metals, especially those of lower atomicnumber such as aluminum, magnesium and boron; carbon; and metal hydridessuch as lithium aluminum hydride.

Minor amounts of other additives such as stabilizers, antacids,catalysts, and the like also may be included in composite propellants ofthe present invention.

It is to be understood that each of the various types of compoundingingredients can be used singly, or mixtures of various ingredientsperforming a certain function can be employed.

The following examples are illustrative of the invention, but are not tobe regarded as limiting the scope of the invention in any way. In theexamples, parts are by weight unless otherwise indicated.

Example 1 This example illustrates the preparation of a thermoplasticelastic copolymer suitable for use in the instant invention.

A mixture of 500 parts of polytetramethyleneether glycol having anaverage molecular weight of about 1000 and parts of 1,4-butanediol isadded dropwise to 1400 parts of liquified phosgene, and held underreflux for 12 hours. At the end of this time, the excess phosgene isremoved by passing a rapid flow of nitrogen gas through the reactionmixture.

The above phosgenation product (50 parts) and 5.8 parts of adipoylchloride is added rapidly to a solution of 11.7 parts of piperazine in540 parts of methylene chloride. The mixture is stirred in a blender athigh speed for one minute and then 200 parts of aqueous sodium carbonatesolution is added, and agitation is continued for minutes. About 0.6part of N,N'-di-B- naphthyl-p-phenylene diamine is added as antioxidantand stabilizer and mixed into the reaction mass. The resulting thickemulsion is poured into boiling water to volatilize and remove themethylene chloride. The insoluble polymer separates as a finely dividedmaterial which is washed with fresh water, and dried.

The thermoplastic elastic copolymer prepared in this way has an inherentviscosity of about 2.5 in m-cresol solution. The copolymer may bepress-molded into a sheet or other shape at about 225 C. and 500 psigpressure, with cooling of the mold to a temperature below 150 C. beforepressure is released. The copolymer also may be injection molded. Usingconventional equipment, the solid molded copolymer is cut into particlesof controlled size which may be preferred for the formulation ofcomposite propellant compositions.

Example 2 This example illustrates formation of a nitroglycerinplasticized binder composition of the instant invention.

Two equal portions of the dried thermoplastic elastic copolymer ofExample 1 in finely divided form were mixed separately at ambient roomtemperature with 1 /2 times and 2 times their weight, respectively, ofnitroglycerin. In each instance, the nitroglycerin was absorbed rapidlyby the copolymer giving a non-flowable composition. After three days ofstorage at room temperature, the mixtures were essentially homogeneousand formstable, and were less tough than a correspondingnitrocellulose-nitroglycerin composition.

Absorption of nitroglycerin occurred much less rapidly when thethermoplastic elastic copolymer was in a more massive form. Thus, twosamples of press-molded copolymer of Example 1 in the form of chipsabout 1 inch square and inch thick were immersed in nitroglycerin atabout C. for six days. The gain in weight, i.e., nitroglycerin absorbed,is shown 'in the tabulation below.

Percent Weight Increase The above data emphasize the importance ofparticle size of the copolymer in controlling the rate of absorption ofnitroglycerin. In 6 days the large chips absorbed less nitroglycerinthan was absorbed almost at once by the same copolymer in finely dividedform. The average degree of polymerization as characterized by theinherent viscosity also is a factor which influences the rate ofabsorption of nitroglycerin to form a nitroglycerinplasticized copolymerof the instant invention. In general, the particle size of the binderwill range from microns to 1000 microns, material of 200 to 500 micronsbeing preferred. The inherent viscosity of the copolymers in m-cresolwill range from 1.7 to 6.0 (believed to correspond to molecular weightsof 50,000200,000), with an indicated preference for copolymers havinginherents in the range of 2.0 or more.

Example 3 A composite propellant composition is made by blending 18.4parts of granular thermoplastic elastic copolymer, prepared as inExample 1, with 63.2 parts of granular ammonium perchlorate. The blendthen is mixed with 18.4 parts of liquid nitroglycerin. The resultingcomposition has a calculated specific impulse of 244 lb. sec./lb.(shifting equilibrium, 1000 psi. to l at-ms.), and a correspondingdensity impulse of 397 see/cc. (density impulse is the product ofdensity and specific impulse).

The specific impulse values in this and the subsequent example are basedon the most recent Joint Army-Navy- Air Force thermodynamic data.

Example 4 A composite propellant containing finely divided aluminum as afuel additive is formulated to have the following composition:

Ingredient: Percent Ammonium perchlorate 69.3 Thermoplastic elasticcopolymer (Ex. 1) 5.0 Aluminum powder 20.7 Nitroglycerin 5 .0

The resulting propellant composition has a calculated specific impulseof 259 lb. sec./lb. (shifting equilibrium), and a corresponding densityimpulse of 506 g. sec/cc.

The gain in specific impulse which results by including the aluminumfuel additive is apparent. The effectiveness of the NGplasticized-thermoplastic elastic copolymer binder of the instantinvention facilitates the formulation of such higher specific impulsepropellants.

It will be understood that numerous changes and modifications may bemade in the subject matter described above without departing from thescope of the invention as defined in the appended claims.

What is claimed is:

1. A composition of matter comprising a mixture of nitroglycerin and athermoplastic elastic copolymer consisting essentially of the recurringunits (a) OGO-, (b) O-XO, and (c) Y connected by the bivalent acylradical wherein OG-O is a bivalent radical obtained by removing thehydroxyl hydrogen atoms from a polymeric glycol having a molecularweight of at least 800 and selected from the group consisting ofpolyalkyleneether glycols and polyester glycols; OXO is a bivalentradical obtained by removing the hydroxyl hydrogen atoms from a glycolhaving a molecular weight of less than 250; and Y is a bivalenthydrocarbon radical having a molecular weight of less than 200 andselected so that the polypiperazine amide, having repeating units of thestructure lilN N t. L J.

in the fiber-forming molecular weight range, melts above 200 C.; saidrecurring units (a), (b), and (0) being present in amounts such that themolar ratios of a:b:c are 1.0:0.03.0:0.0-2.0 and the sum of b+2c isequal or greater than 2.

2. A propellant composition comprising 50 to Weight percent of a solidinorganic oxidizer and from 10 to 50 weight percent of a binderconsisting essentially of a mixture of 10 to 70% by weight ofnitroglycerin and 30 to 90% by weight of a thermoplastic elasticcopolymer consisting essentially of the recurring units (a) OG-O, (-b)-OXO, and (c) Y connected by the bivalent acyl radical CHr-CHz O 7wherein -OGO is a bivalent radical obtained by removing the hydroxylhydrogen atoms from a polymeric glycol having a molecular weight of atleast 800 and selected from the group consisting of polyalkyleneetherglycols and polyester glycols; OXO is a bivalent radical obtained byremoving the hydroxyl hydrogen atoms from a glycol having a molecularweight of less than 250; and -Y--- is a bivalent hydrocarbon radicalhaving a molecular weight of less than 200 and selected so that thepolypiperazine amide, having repeating units of the structure it f e iti CN NOY L GET-CH2 L in the fiber-forming molecular weight range, meltsabove 200 C.; said recurring units (a), (b), and (c) being present inamounts such that the molar ratios of a:b:c are l.0:0.03.0:0.02.0 andthe sum of b+2c is equal to or greater than 2.

3. A propellant composition comprising 50 to 90 weight percent of asolid inorganic oxidizer, 20 to 35 weight percent of a finely dividedmetal, and 10 to 30 Weight percent of a binder consisting essentially ofa mixture of 10 to 70% by weight of nitroglycerin and 30 to 90% byweight of a thermoplastic elastic copolymer consisting essentially ofthe recurring units (a) -OGO, (b) -OXO, and (c) -Y- connected by thebivalent acyl radical wherein O-GO is a bivalent radical obtained byremoving the hydroxyl hydrogen atoms from a polymeric glycol having amolecular weight of at least 800 and selected from the group consistingof poly alkyleneether glycols and polyester glycols; OXO is a bivalentradical obtained by removing the hydroxyl hydrogen atoms from a glycolhaving a molecular weight of less than 200 and --Y-- is a bivalenthydrocarbon radical having a molecular weight of less than 200 andselected so that the polypiperazine amide, having repeating units of thestructure in the fiber-forming molecular weight range, melts above 200C.; said recurring units (a), (b), and (c) being present in amounts suchthat the molar ratios of azbzc are 1.0:0.0-3.0:0.0-2.0 and the sum ofb+2c is equal to or greater than 2.

4. A process for the preparation of the composition described in claim 1wherein the thermoplastic elastic copolymer in the form of fine granulesis mixed with nitroglycerin at room temperature, the mixture is castinto a mold, and the mixture in the mold is heated until plasticization,gelation, and solidification occur.

5. A process for preparation of the composition described in claim 2wherein an intimate mixture of solid inorganic oxidizer and finegranules of thermoplastic elastic copolymer is mixed with nitroglycerin,the resulting mixture is cast into a mold, and the charged mold isheated until plasticization, gelation, and solidification occur.

6. A process for preparation of the composition described in claim 3wherein an intimate mixture of solid inorganic oxidizer, fine granulesof the thermoplastic elastic copolymer, and a finely divided metal ismixed with nitroglycerin, the resulting mixture is cast into a mold, andthe charged mold is heated until plasticization, gelation, andsolidification occur.

No references cited.

LEON D. ROSDOL, Primary Examiner.

OSCAR R. VERTIZ, Examiner.

B. R. PADGETI, Assistant Examiner.

2. A PROPELLANT COMPOSITION COMPRISING 50 TO 90 WEIGHT PERCENT OF ASOLID INORGANIC OXIDIZER AND FROM 10 TO 50 WEIGHT PERCENT OF A BINDERCONSISTING ESSENTIALLY OF A MIXTURE OF 10 TO 70% BY WEIGHT OFNITROGLYCERIN AND 30 TO 90% BY WEIGHT OF A THERMOPLASTIC ELASTICCOPOLYMER CONSISTING ESSENTIALLY OF THE RECURRING UNITS